The present invention relates to a method for manufacturing a semiconductor device, and more particularly to a method for wet-etching a metal oxide film to be a gate insulating film having a high dielectric constant.
A silicon oxide film (SiO2 film) has been used as a gate insulating film of an MIS semiconductor device. On the other hand, the degree of integration of semiconductor integrated circuits has been increased significantly in recent years. When a super thin silicon oxide film having a thickness of about 2 nm or less is used as a gate insulating film, the gate-leak characteristics deteriorate due to a direct-tunneling effect, etc., whereby it is difficult to realize an LSI having a low power consumption.
A high dielectric constant (i.e., high-k) metal oxide film made of an oxide of a metal such as hafnium is expected as a next-generation gate insulating film that replaces a silicon oxide film. For example, when a metal oxide film of hafnium (HfO2 film), having a relative dielectric constant of about 20, is used as a gate insulating film, an HfO2 film can have a capacitance equal to or greater than that of an SiO2 film even if the SiO2-equivalent thickness of the HfD2 film is 2 nm or less. Moreover, by using an HfO2 film as a gate insulating film, it is possible to realize a transistor in which the leakage current is reduced by three orders of magnitude or more from that when an SiO2 film is used.
An HfO2 film is normally deposited by using a sputtering method, a CVD (chemical vapor deposition) method, or the like. An HfD2 film immediately after deposition (i.e., an as-deposited HfO2 film) is easily dissolved by a dilute hydrogen fluoride (DHF) solution. Therefore, a DHF solution can be used as an etching liquid (hereinafter referred to also as xe2x80x9cchemical solutionxe2x80x9d) for wet-etching an HfO2 film. However, when a deposited HfO2 film is annealed, the solubility of the HfO2 film to a DHF solution decreases (J. J. Chambers, et al., Effect of Composition and Post-Deposition Annealing on the Etch Rate of Hafnium and Zirconium Silicates in Dilute HF, The Electrochemical Society 200th Meeting, San Francisco, U.S.A., September 2001, abs. #1434). It is believed that this is due to the alteration of the surface of the HfO2 film through the post-deposition annealing process.
However, in many cases where an HfO2 film is to be used as a gate insulating film, one may desire to remove the HfO2 film by wet-etching after the post-deposition annealing process.
In view of this, the present inventors examined how the thickness of an annealed HfO2 film changes when immersed in various chemical solutions in search for chemical solutions with which an annealed HfO2 film can be removed by wet-etching. The results are shown in FIG. 1, in which xe2x80x9cTimexe2x80x9d denotes the amount of time for which an HfO2 film is immersed in a chemical solution, and xe2x80x9cxcex94xe2x80x9d denotes the change in the thickness of an HfO2 film. Herein, xe2x80x9cxcex94xe2x80x9d being a negative value means that the thickness of the HfO2 film increased. Chemical solution names xe2x80x9cAFxe2x80x9d, xe2x80x9cUPSxe2x80x9d and xe2x80x9cPS etching liquidxe2x80x9d denote xe2x80x9cammonium fluoridexe2x80x9d, xe2x80x9chydrogen peroxide solutionxe2x80x9d and xe2x80x9cpolysilicon etching liquidxe2x80x9d, respectively. The concentration of each chemical solution is shown in % by volume except for KOH. The concentration of xe2x80x9cHF+NH4OHxe2x80x9d being 1% means that the stock (undiluted) solution as shown in the remarks column was diluted to 1% by volume with pure water. The mixing ratio of xe2x80x9cPS etching liquidxe2x80x9d shown in the remarks column is a volume ratio. Finally, xe2x80x9cpeelingxe2x80x9d in the remarks column means that an underlying layer of an HfO2 film was lifted off.
As is apparent from FIG. 1, in addition to DHF solutions, an annealed HfO2 film is not substantially dissolved in any of various chemicals commonly used in semiconductor device manufacturing processes such as hydrogen fluoride (HF) solutions of various concentrations. Thus, it is difficult to remove an annealed HfO2 film by wet-etching with these various chemical solutions. This is believed to be for the following reason. Typically, a metal oxide film such as an HfO2 film transitions from an amorphous state into a monoclinic crystal state by the post-deposition annealing process. Due to this transition, a passive film of HfO2 is formed on the surface of the HfO2 film, whereby it is difficult to remove the HfO2 film by wet-etching.
It is known in the art that a passive film of HfO2 can only be dissolved by a highly oxidative acid such as hot concentrated sulfuric acid. However, it is not practical to use such an acid in a semiconductor device manufacturing process, and it is therefore very difficult to remove an annealed HfO2 film by wet-etching. Thus, it is expected that the complete removal of an HfO2 film will be a significant problem in using an HfO2 film in a transistor. Moreover, it is believed that oxide films of refractory metals, other than HfO2, such as zirconium (Zr), lanthanum (La), tantalum (Ta), aluminum (Al), etc., show a similar tendency.
The present invention has been made in view of the above, and has an object to make it possible to reliably remove, by wet-etching, an insulative metal oxide film whose surface has been altered through an annealing process, or the like.
The present inventors have conducted a continuous process of trial and error aiming to achieve the object set forth above, and have found that an annealed HfO2 film can be easily removed by wet-etching using a DHF solution, or the like, by exposing the annealed HfO2 film to a plasma before wet-etching the annealed HfO2 film. It is believed that the exposure of an HfO2 film to a plasma gives a plasma damage to a surface portion of the HfO2 film to a depth of about 1 to 3 nm, thereby forming a damaged layer, which is less resistant to wet-etching.
FIG. 2 shows the results of an examination obtained by the present inventors on the plasma treatment time (the amount of time for which a plasma treatment is performed before wet-etching) dependence of the amount of an annealed HfO2 film that is wet-etched with a DHF solution. Herein, a mixed gas of an HBr gas, a Cl2 gas and an O2 gas, and a mixed gas of an HBr gas and a Cl2 gas, were used as plasma gas species. As shown in FIG. 2, the use of either plasma gas species allows an HfO2 film to be wet-etched. In view of this, the present inventors believe that the effect provided by the plasma treatment before wet-etching, i.e., the effect of forming a damaged layer in an annealed HfO2 film, can be obtained irrespective of which plasma gas species is used. Note however that in a case where, for example, an HfO2 film is used as a gate insulating film, and a damaged layer is formed in the HfO2 film (a portion thereof that is located outside a gate electrode) successively after forming the gate electrode by dry-etching, it is preferred to use a plasma gas species with which side-etching of the gate electrode is prevented.
Specifically, a wet-etching method of the present invention, which has been made based on the findings set forth above, includes: a first step of annealing a substrate with a metal oxide film deposited thereon; a second step of exposing a surface of the annealed metal oxide film to a plasma; and a third step of removing, by wet-etching, at least a surface portion of the metal oxide film, which has been exposed to the plasma.
According to the wet-etching method of the present invention, the substrate with the metal oxide film deposited thereon is annealed, and then the surface of the metal oxide film is exposed to a plasma, after which at least a surface portion of the metal oxide film is removed by wet-etching. Specifically, the surface of the metal oxide film, which has been altered through the annealing process to be resistant to wet-etching, is exposed to a plasma. Thus, a damage layer that is less resistant to wet-etching is formed at least in a surface portion of the metal oxide film, Therefore, it is possible to reliably remove, by wet-etching, the damaged layer, i.e., at least the surface portion of the metal oxide film.
Note that since the wet-etchable damaged layer is formed only to a depth of a few nanometers from the surface of the metal oxide film, a portion of the metal oxide film may be left unremoved after wet-etching if the annealed metal oxide film has a large thickness. In other words, the metal oxide film cannot be removed completely. In such a case, it is preferred that the metal oxide film is thinned in advance by etching the metal oxide film with a dry-etching gas, for example, before forming a damaged layer in the metal oxide film through a plasma treatment. In this way, a damaged layer can be formed entirely across the metal oxide film, thereby making it possible to completely remove the metal oxide film by wet-etching. This provides effects such as avoiding the occurrence of a metal contamination due to the metal oxide film residue in the subsequent steps.
In the wet-etching method of the present invention, it is preferred that the second step includes a step of applying a bias power to the substrate.
In this way, ions in the plasma can efficiently reach the substrate, thereby giving a greater plasma damage to the metal oxide film. Thus, it is possible to more easily wet-etch the metal oxide film.
In the wet-etching method of the present invention, it is preferred that the plasma is a plasma made of an HBr-containing gas.
In this way, since an HBr-containing gas has a dry-etching effect on a metal oxide film whose surface has been altered through an annealing process, or the like, it is possible to thin the metal oxide film while forming a damaged layer in the metal oxide film. In this way, a damaged layer can be formed entirely across the metal oxide film, thereby making it possible to completely remove the metal oxide film by wet-etching. Note however that an HBr-containing gas also damages the substrate. Therefore, in a case where the surface of a metal oxide film is exposed to a plasma made of an HBr-containing gas before it is wet-etched, it is preferred to wet-etch the metal oxide film after etching the metal oxide film by the plasma treatment to a certain thickness. In this way, it is possible to remove the metal oxide film by wet-etching while reducing the damage to the substrate.
In the wet-etching method of the present invention, it is preferred that the third step is performed by using a fluorine-containing solution.
In this way, the metal oxide film can be removed reliably through the wet-etching process.
In the wet-etching method of the present invention, the metal oxide film may be an oxide film containing at least one of hafnium, zirconium, lanthanum, tantalum and aluminum.
A first method for manufacturing a semiconductor device of the present invention includes: a first step of annealing a substrate with a metal oxide film deposited thereon; a second step of depositing a conductive film on the annealed metal oxide film; a third step of patterning the conductive film so as to form a gate electrode while exposing a portion of the metal oxide film that is located outside the gate electrode; a fourth step of exposing a surface of the exposed portion of the metal oxide film to a plasma; and a fifth step of removing, by wet-etching, the exposed portion of the metal oxide film, which has been exposed to the plasma.
According to the first method for manufacturing a semiconductor device, the wet-etching method of the present invention is used for removing a portion of the metal oxide film to be a high-k gate insulating film that is located outside the gate electrode, whereby the unnecessary portion of the metal oxide film can be removed completely. Therefore, it is possible to reliably prevent a contamination from occurring due to a metal that is included in the metal oxide film in subsequent steps of the process. Therefore, even when a high-k metal oxide film, instead of a silicon oxide film, is used as a gate insulating film in a conventional MOS process, it is possible to manufacture a reliable semiconductor device with a high yield.
Note that in the first method for manufacturing a semiconductor device, the conductive film to be the gate electrode may be, for example, a layered structure of a metal film and a silicon film. In such a case, the conductive film may be patterned by dry-etching while changing the etching conditions as necessary according to the material to be etched. In a case where a silicon film (polysilicon film) is used in a part or whole of the gate electrode, it is necessary to prevent a side surface of the polysilicon film that is forming the gate electrode from being etched in a plasma treatment for altering the surface of a metal oxide film. Therefore, it is preferred that the gas used in such a surface-altering plasma treatment does not contain oxygen.
Moreover, in the first method for manufacturing a semiconductor device, it is preferred that the third step includes a step of successively plasma-etching the conductive film and the metal oxide film by using a mask pattern that covers a gate electrode formation region so as to thin the portion of the metal oxide film that is located outside the gate electrode.
In this way, the unnecessary portion of the metal oxide film is thinned so that the damaged layer can be formed entirely across the unnecessary portion, whereby the unnecessary portion can be completely removed by wet-etching.
A second method for manufacturing a semiconductor device of the present invention includes: a first step of forming a dummy gate electrode on a substrate; a second step of forming an insulative sidewall on a side surface of the dummy gate electrode; a third step of forming an interlayer insulating film on the substrate, on which the dummy gate electrode and the sidewall have been formed, so that an upper surface of the dummy gate electrode is exposed; a fourth step of removing the dummy gate electrode so as to form a recess in the interlayer insulating film with the sidewall being a wall surface of the recess; a fifth step of depositing a metal oxide film on the interlayer insulating film so that the recess is partly filled; a sixth step of annealing the substrate with the metal oxide film deposited thereon; a seventh step of depositing a conductive film on the annealed metal oxide film so that the recess is completely filled; an eighth step of removing a portion of the conductive film that is located outside the recess so as to form a gate electrode in the recess while exposing a portion of the metal oxide film that is located outside the recess; a ninth step of exposing a surface of the exposed portion of the metal oxide film to a plasma; and a tenth step of removing, by wet-etching, the exposed portion of the metal oxide film, which has been exposed to the plasma.
According to the second method for manufacturing a semiconductor device, the wet-etching method of the present invention is used for removing a portion of the metal oxide film to be a high-k gate insulating film that is located outside the recess for forming the gate electrode therein in a process of forming a replacement-type MIS transistor, whereby the unnecessary portion of the metal oxide film can be completely removed. Thus, it is possible to reliably prevent a contamination from occurring due to a metal that is included in the metal oxide film in subsequent steps of the process. Therefore, even when a high-k metal oxide film, instead of a silicon oxide film, is used as a gate insulating film, it is possible to manufacture a semiconductor device including a reliable replacement-type MIS transistor device with a high yield.
In the second method for manufacturing a semiconductor device, the first step may include a step of forming a dummy gate insulating film between the substrate and the dummy gate electrode, and the fourth step may include a step of removing the dummy gate insulating film.
In the second method for manufacturing a semiconductor device, it is preferred that the ninth step includes a step of plasma-etching the exposed portion of the metal oxide film so as to thin the exposed portion of the metal oxide film.
In this way, the unnecessary portion of the metal oxide film is thinned so that the damaged layer can be formed entirely across the unnecessary portion, whereby the unnecessary portion can be completely removed by wet-etching.